This invention relates to an apparatus and a method for forming a thin film on a substrate surface, and particularly to those for vapor-phase deposition of a thin film using a liquid reaction material having a low vapor pressure.
Heretofore, in production processes of semiconductor circuits or liquid crystal circuits, a chemical vapor deposition (CVD) method has been employed for forming various thin films. In the CVD method, plasma excitation energy or thermal energy is exerted onto material gas necessary for forming a film so as to form a thin film on a base plate or substrate. Recently, in view of demands for more integrated semiconductor circuits, as a reaction material, what is used is liquid reaction materials having a low vapor pressure, and which have good characteristics of covering steps and filling in spaces.
A method of introducing the liquid reaction material into a reaction chamber upon vaporization of the material includs a bubbling method wherein the liquid material is vaporized by passing inert gas thereinto, a baking method wherein the liquid material is vaporized directly by heating, a direct vaporization method wherein the liquid material is vaporized by introducing it along with a carrier gas to a heated and pressure-reduced space, an ultrasonic wave method wherein the liquid material is vaporized by heating mist of the material generated by ultrasonic wave vibration, and so forth.
However, by these conventional methods, a constant amount of reaction gas cannot be provided for the reasons below, and further, a large amount of low vapor pressure liquid reaction material cannot be provided, thereby causing drawbacks of decreasing film reproducibility and deposition rate.
That is, in the above bubbling method, in order to maintain the flow of vaporized liquid material at a constant rate, it is necessary to keep the temperature of the liquid material constant. However, the temperature of the liquid material gradually decreases due to the latent heat upon its vaporization by the carrier gas, and thus it is technologically difficult to maintain a temperature constant during the vaporization process.
Further, in the bubbling method and in the direct vaporization method, the amount of carrier gas needs to be increased in order to obtain a large amount of liquid material gas. However, even if the absolute amount of the liquid material gas is increased, the concentration of the liquid material gas is decreased, thereby subsequently reducing the speed of film formation.
In the baking method, in order to obtain a large amount of liquid material gas, it is necessary to heat the liquid material to a high temperature by using a large and expensive heat chamber. However, this method cannot be applied to liquid material which is prone to decompose or deteriorate with heat. Further, it is necessary to heat the entire passage of the liquid material gas in order to prevent re-liquification of the liquid material gas, and thus in view of problems in heat resistance of the devices and the cost of the apparatus, high-temperature operation may be barred to a certain extent.
Additionally, in the ultrasonic wave method, liquid material apt to decompose or deteriorate by heat cannot be used. Further, when liquid material which is difficult to vaporize is used, it is necessary to heat the liquid material for vaporization. In this case, heat resistance of the ultrasonic vibrator becomes an issue.